1. Field of the Invention
The present invention relates to a digital imbalance correction method and device. In particular, the present invention relates to such a device adapted for use in a receiver designed for multi-carrier applications.
2. Description of the Related Art
With the recent progress in telecommunications, receivers tend to be designed for so-called multi-carrier applications in order to be able to receive signals composed of multiple carriers, each carrier originating from a different transmitter. For example, such applications are likely to occur in connection with e.g. frequency diversity scenarios.
This invention relates to such broadband dual (or multi-) branch receivers for such multi-carrier applications. A broadband dual (or multi-) branch converter is used to convert the complete RF band (Radio Frequency) of interest including multiple channels/carriers to a low pass limited frequency band. This low-pass limited spectrum will be converted into digital data by low-pass ADCs (Analog-to-Digital Converters) after the analog branches.
Precisely, this represents a low IF receiver (Intermediate Frequency), as each independent channel is still on an IF frequency. The term “Direct Conversion” is applied here to emphasize that a complete RF band is converted to the baseband by means of an I/Q (or multi-branch) converter, lowpass sampled and converted to a digital signal. This digital signal typically contains several independent channels, which are then separated by digital filters. Alternatively, it contains one broadband channel such as an OFDM (Orthogonal Frequency Division Multiplexing) or an arbitrary signal.
FIG. 1 shows a basic analog part of an I/Q conversion receiver. An input RF signal is supplied to an input terminal 1. The input RF signal contains one or more individual channels (multi-carrier signal) and may originate from several independent transmitters. Multi-branch receiver (RX) means that there is one antenna and in principle only one receiver, but the receiver has several parallel branches to process the signal from one antenna. That is typically the direct conversion receiver. In a superhet receiver, the image frequency is suppressed by a filter. In the Direct Conversion receivers, the image is too close to be filtered. The two branches allow to separate the images, although the contents in each branch is corrupted. Adding a third or more branches with different phases would allow to correct for hardware errors (like DC offset). In short, diversity copes with path distortion, multi-branch copes with hardware distortion.
Herein below, “branch” is used for denoting a hardware branch and/or signal path, whereas “component” (such as I and Q component) is used to denote a mathematical descriptive model of a signal. Stated in other words, a signal can be mathematically described by its components (I and Q components), the two components may be processed digitally: if processed in serial, one hardware branch is needed therefor, while if processed in parallel, two (or more) hardware branches are required. Thus, the analog RF front-end shown in FIG. 1 has two independent branches (or path for a signal component), one for the Q- and one for the I-component.
A bandpass filter BPF 2 following the RF input terminal 1 selects a certain frequency band. The thus selected frequency band is amplified using a low noise amplifier LNA 3, and supplied to a power splitter 4 which splits the signal in two branches. Each respective branch of the split signal is supplied to a respective mixer 5a, 5b, respectively. At the respective mixer 5a, 5b, the split signal component (carried in the concerned branch) is subjected to a mixing using a respective signal supplied from a local oscillator 6. The local oscillator 6 generates two oscillating output signals mutually shifted by 90° such as a sine and a cosine signal, using a phase locked loop PLL 6b and a oscillator element 6a. The mixed signals are respectively subjected to a subsequent low pass filtering using low pass filters LPF 7a, 7b, amplified by amplifiers 8a, 8b, respectively, and finally output by a respective intermediate low pass filter LPF 9a, 9b. These outputs are designated as an I output 10a and a Q output 10b, respectively. The I output 10a represents the in-phase component of the signal, whereas the Q output represents the quadrature component of the signal.
The low pass filters 7a, 7b, respectively, downstream the mixers 5a, 5b select a wanted channel (or wanted channels, depending on the width of the passband of the filters) within the band remaining after the filtering by BPF 2. The dual branch or I/Q receiver as in FIG. 1 is known per se, and an example for TV applications is disclosed in the U.S. Pat. No. 4,633,315.
One of the key impairments of this architecture resides in an amplitude (gain) and phase imbalance of the two (or multiple) signal branches in the receiver. In a single carrier receiver (RX) this I/Q-imbalance reduces the signal-to-noise ratio S/N of the receiver and causes performance degradation.
Efforts are therefore made in order to correct for such an I/Q imbalance. Gain and phase correction in a dual branch receiver is for example disclosed in the European patent EP-A-0 305 603 and the U.S. Pat. No. 6,044,112. These prior art patents, however, are related to true direct conversion receivers for single carrier applications.
In the case of multi-carrier reception, however, amplitude and phase imbalance of the two branches (i.e. I and Q branch) cause interference of the channels located at equal frequency differences above and below the frequency of the local oscillator 6. Phase and amplitude differences of the I- and Q- branches limit the practical image rejection to some 30 dB, while for a GSM multi-carrier application more than 65 up to 95 dB of image rejection is required. Note that image rejection is achieved by summing the I/Q-signals with proper phase and amplitude. The achievable image rejection can be directly calculated from amplitude and phase mismatch (imbalance) of the two branches (I,Q), which in turn are directly related to component tolerances.
In prior art, in order to deal with problems in connection with multi-radio/multi-channel arrangements, there are basically two solutions:                A single conversion to a high IF and IF sub-sampling. Currently available analog-to-digital converters (ADCs) are, however, not sufficient and immense effort is spent by various manufactures to overcome this problem.        A double conversion to a low IF and low-pass sampling. This, however, requires enormous linearity of the RF stages. This solution is on the very edge of being feasible with currently available technology. Although it seems to be feasible, it is associated with rather high implementation costs. Furthermore, the double IF solution is prone to spurious responses and requires different IF frequencies for different RF bands.        
Furthermore, I/Q-error correction is being investigated, and multiple methods have been published on I/Q-error correction. However, these papers concentrate on correction algorithms for single channel or OFDM applications, where the band of interest comes from one single signal source. Multi-carrier applications, however, where each carrier originates from a different transmitter (TX) have to handle a considerable higher dynamic range. In a disseration by Mikko Valkama, Tampere University of Technology, 2001, it is discussed that the task of improving the image signal attenuation of the basic quadrature down-conversion scheme, either using analog or digital techniques, has been addressed to some extent in recent literature, where several different ideas are discussed. Commonly, in the digital methods, the approach is to estimate the effective mismatches between the I and Q branch amplitudes and phases. Then, employing these estimates, some kind of a correction network is used to restore the ideal matching conditions (equal amplitudes and a phase difference of 90°). However, most of these methods share the problem of being unable to compensate for amplitude and phase mismatches which depend on frequency and/or time. Furthermore, most of the proposed estimation techniques are based on known test or calibration signals, thereby complicating their use during the normal receiver operation.
Consequently, the drawbacks of the known methods are one or a combination of the following:                Not able to compensate frequency dependent mismatches        Not able to compensate time dependent mismatches        Need for test or calibration signals.        
Even though M. Valkama mentions in his thesis and further publications “Statistical Signal Processing Techniques for
Imbalance Compensation”, these are based on using certain assumptions on the wanted and interfering signals, in order to be able to compensate the amplitude and phase mismatches without needing test signals and also compensating for frequency and time dependencies. However, in order to function properly, the assumed frequency dependency model has to be correct. Otherwise, the desired result can not be obtained. However, the estimation of time dependency is also a problem in case of fast varying signals, such as in the GSM system.
In summary, direct conversion of a multi-carrier signal, however, is currently not feasible for cellular applications.